In a radio-frequency transceiver, mismatches between in-phase (I) and quadrature (Q) baseband signal paths result in amplitude and phase errors that degrade the quality of the transmitted or received signal. In a transceiver without calibration, amplitude and phase errors up to 0.3 dB and 3°, respectively, can occur. This is acceptable for many radio frequency (RF) communication systems, such as wireless local area networks (LANs) based on the IEEE standard 802.11b. However, in order to achieve an acceptable error vector magnitude for the IEEE wireless LAN standards 802.11a and 802.11g, the transceiver must achieve ≦0.1 dB amplitude error and ≦1° phase error, thereby requiring some form of calibration.
The amplitude and phase errors of an integrated transceiver can be corrected either by an end-of-line calibration or by self-calibration. End-of-line calibration is a one-time calibration performed at the end of the manufacturing process of the transceiver chip or of the circuit board on which it is mounted and generally requires the use of circuitry external to the transceiver. For self-calibration, the calibration is performed repeatedly throughout the operating lifetime of the transceiver chip or circuit board, and the entire calibration circuitry is on-chip. In addition to being either end-of-line calibration or self-calibration, calibration can be performed with either a continuous-wave (CW) signal or a modulated signal.
Compared to an end-of-line calibration, self-calibration has the advantages that the calibration does not increase the cost or duration of the manufacturing process or require additional equipment. In addition, self-calibration avoids the need for a non-volatile memory to store the calibration settings, and it automatically corrects any drift of the amplitude and phase errors during the operating lifetime of the transceiver.
In typical CW calibration procedures, the transmitter is calibrated by applying sinusoidal test signals of equal amplitude and frequency with a 90° phase difference to the baseband inputs of the transceiver, measuring the spectrum of the RF output signal, and adjusting the control inputs for the amplitude and phase balance so as to minimize the amplitude of the suppressed sideband. The receiver is calibrated in an analogous manner by applying a single tone, which is offset from the carrier frequency, to the RF input, measuring the relative amplitude and phase of the resulting sinusoidal signals at the baseband outputs, and adjusting the control inputs such that the amplitudes of the baseband outputs are equal and the phase difference between the baseband outputs is equal to 90°.
In this type of calibration, the control inputs are adjusted in an iterative manner, alternating between amplitude and phase adjustments, until the amplitude of the suppressed sideband at the transmitter output and the amplitude and phase errors at the receiver outputs are within defined limits. This technique becomes too complicated if separate adjustment of the baseband and carrier components of the phase error is needed to achieve desired accuracy. The complication arises from the fact that the measurement procedure must not only alternate between three control inputs (amplitude error, baseband phase error and carrier phase error), but also between upper-sideband (I leads Q) and lower-sideband (I lags Q) test signals.
In principle, after the transmitter is calibrated, the transmitter output could be used as the test signal for the receiver. However, any remaining errors in the transmitter would result in a residual unwanted sideband and degrade the calibration accuracy of the receiver. Thus, in practice, a separate, single-tone RF signal source is needed to calibrate the receiver by the method described above. For self-calibration, implementing such a source on-chip with the necessary frequency accuracy would require an additional phase-locked loop (PLL), which is undesirable.
Thus, there remains a need for a transceiver including self-calibration circuitry that shortens and simplifies the calibration procedure.